The present invention relates to a dispersion compensator suitable for use in an optical communication system, for compensating for wavelength dispersions of a transmission medium such as an optical fiber or the like in an optical pulse transmission path.
In an optical transmission system, a high-purity silica optical fiber is normally used as a transmission medium for a light signal. However, since the optical fiber has a wavelength dispersion, a pulse waveform is degraded when a light signal pulse having predetermined wavelength broadening is transmitted. The degradation of the light pulse waveform due to the dispersion of the optical fiber becomes a big factor that will restrict a transmission distance and transmission capacity of the optical transmission system. Therefore, the technology of canceling out such a wavelength dispersion becomes important for the large-capacity optical transmission system. If, for example, an optical system having the dispersion of the optical fiber and its reverse dispersion is inserted into an optical transmission path, then the dispersions of the optical fiber are canceled out so that the degraded waveform can be recovered or made up.
As the prior art, a technology for compensating for dispersions by use of a fiber (dispersion compensating fiber) having dispersions reverse in sign and large in absolute value has been put into practical use. The dispersion compensating fiber has been widely used because it has such characteristics that it is capable of realizing a desired characteristic with satisfactory reproducibility and has a wide compensable band, for example. However, a dispersion compensating value per unit length of the dispersion compensating fiber is as low as about xe2x88x9220 ps/nm/km. If one attempts to obtain a desired dispersion value, then a very long fiber is needed. Therefore, a problem arises in that it cannot be brought into less size and becomes high in cost.
As a recent technology which aims to scale down a dispersion compensator, there has been proposed a dispersion compensator using a multidimensional structure of two or more types of mediums different in refractive index, i.e., a photonic crystal. It is known that light transmitted through the photonic crystal exhibits a peculiar dispersion characteristic. When a suitable lattice structure, a cycle or period, and the difference in refractive index between the mediums are selected with respect to light having a desired wavelength, a large dispersion can be obtained. A specific example thereof has been disclosed in Japanese Patent Application Laid-Open No. 2000-121987. The present dispersion compensator is one wherein a two-dimensional photonic crystal is formed on an Si (Silicon) substrate to compensate for dispersion. The dispersion compensator is capable of obtaining a few tens of +ps/nm with a length of 5 mm.
However, the light that propagates through the two-dimensional photonic crystal, cannot avoid losses produced due to it scattering. The example disclosed in Japanese Patent Application Laid-Open No. 2000-121987 did not taken into consideration the insertion losses. Further, when the dispersion relations of the photonic crystal are used, a part of a complex dispersion curve is locally used. An extremely high degree of production accuracy is required to obtain desired performance, and the degree of freedom of design of the dispersion compensator is limited.
As described in the prior arts, there are known ones in which dispersion compensation for long-distance optical fiber communications has already been put into practical use. However, a dispersion compensator small in size and low in cost has not yet been realized.
With an increase in transmission capacity, there has been a need to use a large number of channels in higher density and over a wide range of wavelength regions or bands. Correspondingly, there is a need to assure wavelength dispersions more accurately. Further, a problem arises in that wavelength dispersions change with time due to a change in physical-property constant or the like with a variation in outside-air temperature. However, the conventional dispersion compensating system provides a fixed compensable dispersion value and is hence not capable of compensating for wavelength dispersions in an optical fiber, which change momentarily. In order to cope with such a problem, there has been a demand for a dispersion compensator capable of changing a compensation value flexibly and accurately according to conditions.
Thus, an object of the present invention is to provide a dispersion compensator which is ultra small in size and low in cost, and is capable of changing a dispersion compensating value, and an optical transmission system using the same.
In a dispersion compensator according to the present invention, defect modes of a photonic crystal are used to configure a waveguide having coupled microcavities or resonators, and a dispersion property of light that propagates through such a waveguide, is used to compensate for each waveform dispersion.
Upon the occasion of description of the principle and effects of the present invention, a description will first be made of the principle of allowing each defect in photonic crystal to function as a microcavity. Next, the concept and propagation characteristic of the waveguide having the coupled microcavities will be demonstrated and the principle of dispersion compensation using it will be explained.
The photonic crystal is a multidimensional periodic structure comprising combinations of two or more mediums different in refractive index. FIG. 2 shows an example of one called a xe2x80x9ctwo-dimensional photonic crystalxe2x80x9d of photonic crystals. FIG. 2 is a cross-sectional view of a structure which includes periodic structures as viewed in the direction horizontal to the sheet and vertically-extending structures are uniform. Columns each having a dielectric constant xcex52 are disposed in a medium having a dielectric constant xcex51 in a triangular lattice form (xcex51 greater than xcex52). When each columnar portion is hollow, xcex52=1. In FIG. 2, a indicates a lattice constant, and r indicates the radius of each column.
A diagram showing the relationship between a wave number of light propagating through a photonic crystal and the frequency thereof is called a photonic band chart. FIG. 3 is a photonic band chart relative to a TM mode when xcex51=3.5, xcex52=1 and r/a=0.45 in the structure shown in FIG. 2. Here, the TM mode indicates a mode in which an electric field is vertical to the sheet. The vertical axis indicates a normalized frequency (xcfx89a/2xcfx80c), and the horizontal axis indicates a wave vector (ka/2xcfx80) normalized within a first brillouin zone. c indicates a light velocity in vacuum, xcfx89 indicates an angular frequency of the light and k indicates a wave number, respectively. The triangular lattice of FIG. 2 corresponds to a hexagonal symmetry. A formed brillouin zone is an orthohexagonal structure shown in FIG. 3. The apex of the orthohexagon is a point K, the midpoint of each side is a point M, and a point where a wave number is 0, is a point xcex93, respectively.
As diagonally shaded in FIG. 3, no band exists in a specific (normalized) frequency region over the whole region or band of the first brillouin zone. This means that light having a frequency corresponding to this band cannot propagate through a photonic crystal. Such a frequency band in which the propagation is prohibited, is called a xe2x80x9cphotonic bandgapxe2x80x9d. For example, when light having a wavelength corresponding to a bandgap is launched into a crystal from outside, it is fully reflected.
Now consider where point defects, i.e., ununiform elements in periodic structures are introduced into a photonic crystal having a bandgap. Since the periodic structures are out of order at the defective portions, the band chart shown in FIG. 3 is not applied and even light having a bandgap wavelength can exist. Since, however, the periphery of the defect is of the perfect photonic crystal, the light cannot propagate toward the outside and is hence reflected toward the interior of the defect. FIG. 4 conceptually shows the manner at this time. On the other hand, a point defect and a photonic crystal lying therearound form a microcavity. The light is multi-reflected thereinside and placed in a confined state, so that it forms a steady state. The steady state of the light within the defect of the photonic crystal is called a xe2x80x9cdefect levelxe2x80x9d.
Since there is a need to fully reflect the light by the peripheral photonic crystal with a view toward operating each defect as the microcavity, the defect level always depends on the frequency corresponding to the bandgap. As a specific form of each defect, there is generally known such a form that the diameter of a column (or hole) having xcex52 is changed. In the defect shown in FIG. 4, the diameter of one column can be regarded as 0. However, even if the diameter thereof is made large, the defect level is formed. In either case, no specified process technology is required for the fabrication of each microcavity so long as the technology of forming the photonic crystal is adopted. A plurality of microcavities can also be fabricated on the same plane.
If the defect of the photonic crystal is used as described above, then each microcavity simple in structure can be fabricated with relative ease and arbitrarily in regard to the density and position. Such a feature is extremely suitable for the fabrication of coupled-microcavity waveguide to be described below.
The characteristic of the coupled-microcavity waveguide has been described in, for example, xe2x80x9cOptics Letters, Vol. No. 24, pp. 711xe2x80x9d. FIG. 5 typically shows the manner of the propagation of light used therein. As shown in the drawing, the coupled-microcavity waveguide is a structure wherein microcavities each having a resonant frequency (i.e., frequency of localized mode) xcexa9 are disposed in continuous connection with one another at predetermined intervals xcex9. Normally, the light is repeatedly internally reflected in the microcavities to thereby form a standing wave. When an ideal microcavity exists in isolation, photons are confined thereinside and no leak to the outside.
However, when another microcavity is placed at a suitable distance, a spatial distribution of light confined in a microcavity I and a spatial distribution of light confined in a microcavity II overlap each other. It is therefore possible to transfer energy between the two microcavities. Thus, the transfer of the energy between the microcavities refers to xe2x80x9ctwo microcavities are coupled to each otherxe2x80x9d.
When a large number of microcavities are arranged as shown in FIG. 5 in a state in which the two adjacent to each other are connected to each other, the above-described energy propagation is continuously repeated and an incident pulse propagates through the microcavities one after another. This results in the principle of the coupled-microcavity waveguide.
The propagation characteristic of light in the coupled-microcavity waveguide has been well described by xe2x80x9cTight Binding Approximationxe2x80x9d that light is tight bound with each microcavity and it interacts with only a microcavity adjacent thereto. Assuming that the resonant frequency of each microcavity is defined as xcexa9, and the angular frequency of the light that propagates through the coupled-microcavity waveguide is defined as xcfx89, the following relation is established under the xe2x80x9cTight Binding Approximationxe2x80x9d.
xcfx89=xcexa9(1+xcexa cos(kxcex9))xe2x80x83xe2x80x83(1) 
where xcexa indicates a value related to the strength of the interaction between the microcavities and indicates a constant determined by the structure of the microcavity, the distance between the microcavities, etc. k indicates a wave vector of the light lying in the coupled-microcavity waveguide. It is understood that as represented by the equation (1), xcfx89 takes or assumes each value in a range equivalent to (1xc2x1xcexa) times of xcexa9.
A group velocity Vg of the light that propagates through the coupled-microcavities, is represented by the following equation (2).                                                         Vg              =                              xe2x80x83                            ⁢                                                                    ⅆ                    ω                                    /                                      ⅆ                    k                                                  =                                                      -                    κΛΩ                                    ⁢                                      xe2x80x83                                    ⁢                                      sin                    ⁡                                          (                                              k                        ⁢                                                  xe2x80x83                                                ⁢                        Λ                                            )                                                                                                                                              =                              xe2x80x83                            ⁢                                                ΛΩ                  ⁡                                      (                                                                  κ                        2                                            -                                                                        (                                                                                    ω                              /                              Ω                                                        -                            1                                                    )                                                2                                                              )                                                                    1                  /                  2                                                                                        (        2        )            
It is understood from the equation (2) that the absolute value of the group velocity takes a maximum value xcexaxcex9xcexa9 when xcfx89=xcexa9, whereas when xcfx89=(1xc2x1xcexa)xcexa9, it assumes a minimum value 0. In general, a wavelength dispersion D is defined by the following equation:
D=d(1/Vg)/dxcexxe2x80x83xe2x80x83(3) 
It is understood from the equation (3) that the dispersion is an inclination to a wavelength change in the inverse 1/Vg of the group velocity. The manner in which the inverse 1/Vg of the absolute value of the group velocity is defined as a xcex function, is shown in FIG. 6. xcex0 indicates the wavelength of light when xcfx89=xcfx89. xcex1 and xcex2 indicate light""s wavelengths when xcfx89=(1xc2x1xcexa)xcexa9. As shown in the drawing, 1/Vg results in such a downwardly-extending convex form that it becomes infinity at xcex1 and xcex2 and it takes a minimum value 1/xcexaxcex9xcexa9 at xcex0.
As represented by the graph of FIG. 6, the inclination of 1/Vg extends from minus infinity to 0 and continuously changes up to plus infinity. Thus, when it is applied to the dispersion compensator, it has a feature that a very large dispersion is obtained in the first place and necessary dispersion values can arbitrarily be selected inclusive of the positive and negative of a sign in the second place.
Further, the loss incident to the propagation is not produced in principle. Since the light propagates even in actuality in a state of being tightly confined in a narrow region, the loss thereof due to its scattering or the like is expected to be extremely low. Further, since the light is tightly confined in each microcavity per se, there is no need to newly form a waveguide structure, and a dispersion compensator can be fabricated by a simple process.
The present invention relates to a dispersion compensator which makes use of the dispersion characteristic of light that propagates through each defect in the photonic crystal described above and compensates for wavelength dispersions developed in an optical pulse transmission path. The dispersion compensator according to the present invention is large in wavelength dispersion value in this way and obtains a necessary dispersion compensating value with an extremely small size. The dispersion compensator can be brought into a downscale of a few hundredths or less as compared with a conventional one under a structure excluding a drive unit, etc., for example. Further, an advantage is brought about in that there is no need to provide a light-confined structure with low loss, and a fabrication process is also easily allowed through the use of a normal semiconductor process technology.